When Whales Walked: The Evolution of Cetaceans

The whales are the largest animals that have ever lived on earth. The Blue Whale is over 100 feet long and weighs up to 190 tons–its tongue alone weighs more than an African Elephant, the largest living land animal. But the huge marine whales evolved from a small land-dwelling animal about the size of a large dog.

Zygorhiza, an ancient whale closely related to Basilosaurus, on display at the Smithsonian Museum of Natural History

Until fairly recently, the fossil history of the earliest whales (known as Cetaceans) was quite unknown. Edwin Colbert pointed out in 1955, “These mammals must have had an ancient origin, for no intermediate forms are apparent in the fossil record between the whales and the ancestral Cretaceous placentals. Like the bats, the whales (using the term in a general and inclusive sense) appear suddenly in early Tertiary times, fully adapted by profound modifications of the basic mammalian structure for a highly specialized mode of life.” The oldest whales then known, the Archaeocetes, already exhibited all of the typical whale characteristics, including lack of rear limbs, paddle-like front limbs, and a tail with a horizontal fluke for propulsion.

The first hint of earlier whale ancestry came in 1983, when researcher Phil Gingerich found a 52-million year old skull of a land-swelling mammal in shallow deposits in Pakistan. Although fragmentary, the skull had teeth that were nearly identical with those of the Archaeocetes, and the configuration of the bones at the rear of the skull also matched the Archaeocetes. Gingerich thus concluded that the animal, which he named Pakicetus, was a very primitive whale–a land-dwelling mammal that nevertheless spent a lot of time in the water. “In time and in its morphology,” Gingerich reported, “Pakicetus is perfectly intermediate, a missing link between earlier land mammals and later, full-fledged whales.”

Reconstructed Pakicetus skeleton, on display at the LA County Museum of Natural History.

Despite the whale-like characteristics of the skull, however, Pakicetus lacked two important adaptations which are present in modern whales. In living whales, the ears contain large sinuses that can be filled with blood, allowing the animal to maintain pressure while diving. Modern whales also transmit sound vibrations to the inner ear using a “fat pad”, which allows them to hear directionally underwater. Pakicetus lacked both of these features, indicating that it was unable to dive deeply and that it could not hear well underwater. These anatomical clues meshed well with its habitat, since the Pakicetus bones were found in deposits that had been laid down at the mouth of a river on the shore of a shallow sea, where the opportunities for deep diving would be limited.

Although no post-cranial bones of Pakicetus were found, it seemed logical to assume, from the teeth and ear structure, that the animal spent a great deal of time in shallow water looking for food, but returned to the land to rest, somewhat like a modern sea lion or river otter. It was expected, therefore, that the limb bones of the earliest Archaeocetes would be large and functional, unlike the atrophied remnants of limb bones found in modern whales. (This hypothesis was supported by the fact that several species of modern whales have well-developed rear limbs while embryos, which later atrophy and become nonfunctional).

This conclusion was bolstered in 1990, when Gingerich found another fossil in the Zeuglodon Valley in Egypt. This was a specimen of Basilosaurus, an animal that had originally been uncovered back in 1834, but misidentified as a marine reptile. It was identified as a whale in 1840. Like the earlier specimen, Gingerich’s new Zeuglodon specimen included a four-foot long skull and a number of ribs, all of which bore the unmistakable characteristics of whales, and which matched the corresponding points in the old Pakicetus skull. More importantly, Gingerich also found a femur, a kneecap, both lower leg bones, a number of ankle bones, and three toes, showing beyond any doubt that Basilosaurus had complete rear limbs.

Although the limbs were fully-formed and functional, however, they did not connect to the whale’s pelvic bones, and could not have been used to support the animal’s weight out of the water. Basilosaurus, therefore, was a whale with a prominent rear leg, but one that was so adapted to a marine existence that it could no longer leave the water. This is exactly what paleontologists had predicted they would find in ancient whales.

Gingerich also pointed out, however, that Basilosaurus had a number of unique features in the spinal column which are not shared by modern whales, leading him to believe that, while it was a whale and a descendent of a Pakicetus-like animal, Basilosaurus died out without leaving any modern descendents.

However, the paleontologists predictions concerning a limbed whale-ancestor were further confirmed in 1993, when Gingerich found two new specimens of Indocetus, an early Archaeocete which was intermediate in age between that of Pakicetus and Basilosaurus. The new Indocetus specimens included a piece of pelvic bone, tibia and femur, but no foot bones. Although the find was fragmentary, it was evident from the size and structure of the bones that Indocetus possessed fully-formed functional rear limbs. “The pelvis has a large and deep acetabulum, the proximal femur is robust, the tibia is long . . . All these features, taken together, indicate that Indocetus was probably able to support its weight on land, and it was almost certainly amphibious.” Indicetus had not yet, however, developed the loosely constructed sacral bones that allow the modern whales to swim by flexing their spines up and down–a mode of locomotion which is unique among marine animals.

The most recent spectacular discovery in cetacean evolution was in January 1994, when Hans Thewissen announced the discovery of several 49 million year old Archaeocete skeletons, the most complete one consisting of parts of the skull and jaw, a number of vertebrae, some ribs and nearly complete front and hind limbs. The large limb bones were fully capable of supporting the animal’s weight on land, and were also capable of paddling it through the water using an up-and-down motion of the spine. Thewissen named the animal Ambulocetus natans (“the swimming whale that walks”). In morphology and in timing, it is a perfect intermediary between terrestrial mammals and the younger Archaeocetes.

Ambulocetus was about the size of an adult sea lion, and weighed something around 650 pounds. “Ambulocetus is clearly a cetacean”, Thewissen concluded. In addition to the same type of teeth possessed by all of the Archaeocetes and a small hoof at the end of each toe like terrestrial mammals, Ambulocetus also had the whalelike skull characteristics that are found in the Archaeocetes, including an ectotympanic with a large sygmoid process, a reduced zygomatic arch, a wide supraorbital process and a narrow muzzle. While these characteristics were also present in some terrestrial mammals, Ambulocetus also possessed the small protocones and large accessory cusps which distinguish the whales from other mammals.

The limbs were large and strong, and were capable of carrying the body weight on land. “The skeleton of Ambulocetus indicates that it could locomote on land and in the water. As in extant cetaceans, Ambulocetus swam by means of dorsoventral undulations of its vertebral column, as evidenced by the shape of the lumbar vertebra. Unlike modern cetaceans, however,Ambulocetus had a long tail and thus probably lacked a tail fluke.” Thus, the skeleton ofAmbulocetus demonstrates a mixture of characteristics from the terrestrial animals and those of fully aquatic whales. “As such,” Thewissen concludes, “Ambulocetus represents a critical intermediate between land mamals and marine cetaceans.”

Because of this recent series of discoveries, we now know much more about the process of cetacean evolution than we did fifty years ago. The earliest known cetacean, Pakicetus, demonstrates a mixture of traits which are unique to the terrestrial mammals as well as marine whales, and indicates that the cetaceans are descended from land animals. Although we have not found any post-cranial bones from Pakicetus yet, those which have been found withAmbulocetus demonstrate that the earliest members of the mammal–whale transition were animals which spent large amounts of time on land and in the sea, and presumably had lifestyles somewhat similar to those of modern sea lions.

After Ambulocetus, there is a trend towards increasing specialization for a totally marine existence. The Indocetus fossils show a reduction in the size of the limbs, while theBasilosaurus skeleton shows a reduction in the size of the hind limbs to the point where the animal could no longer leave the water. Finally, the later Archaeocetes show the rear limbs shrinking to near non-existence, along with the development of such cetacean specializations as the pressurized inner ear and the “fat pad”. Taken as a whole, the Pakicetus-Ambulocetus-Indocetus-Basilosaurus series is a perfect vindication of the predictions made by paleontologists fifty years ago.

Reconstructed whale evolution (photo from Tumblr)

So which group of land mammals are the whale’s ancestors? Paleontologists had long thought that the teeth of the Archaeocetes resembled those of an ancient group of carnivores called Mesonychids, which were wolf-sized scavengers that lived in the early Eocene period. Based on these similarities, most paleontologists hypothesized that the whales were the evolutionary descendants of the terrestrial Mesonychid carnivores. But there were also similarities with other mammal groups, including the even-toed “artiodactyls” which include the pigs and cows, and the argument was never settled.

Today, using DNA analysis, we can say with near certainty that the closest living relative of the marine cetaceans is the hippopotamus, and that whales are therefore descended from a common ancestor with the hippo, most likely from an ancient group of even-toed land mammals called anthracotheres, which looked sort of like a multi-horned rhinoceros.